Electrolytic System And Reduction Method For Electrochemical Carbon Dioxide Utilization, Alkali Carbonate Preparation And Alkali Hydrogen Carbonate Preparation

ABSTRACT

The present disclosure relates to electrolysis. The teachings thereof may be embodied in a reduction process and/or an electrolysis system for electrochemical carbon dioxide utilization wherein carbon dioxide is introduced into an electrolysis cell and reduced at a cathode. For example, an electrolysis system for carbon dioxide utilization may comprise: an electrolyzer including an anode in an anode space and a cathode in a cathode space. The cathode space has an entrance for carbon dioxide. The cathode space comprises a catholyte including alkali metal cations. The anode space has an entrance for an anolyte. The anode space comprises an anolyte comprising chlorine anions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2016/065277 filed Jun. 30, 2016, which designatesthe United States of America, and claims priority to DE Application No.10 2015 212 504.1 filed Jul. 3, 2015, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. The teachings thereofmay be embodied in a reduction process and/or an electrolysis system forelectrochemical carbon dioxide utilization wherein carbon dioxide isintroduced into an electrolysis cell and reduced at a cathode.

BACKGROUND

At present, about 80% of the global energy requirement is provided bythe combustion of fossil fuels, the combustion processes of which causeglobal emission of about 34 000 million metric tons of carbon dioxideinto the atmosphere per annum. This release into the atmospherecomprises the majority of carbon dioxide released, which can be up to 50000 metric tons per day in the case of a brown coal power plant, forexample. Carbon dioxide is one of the gases known as greenhouse gases,the adverse effects of which on the atmosphere and the climate are amatter of some dispute. Since carbon dioxide exists at a very lowthermodynamic level, it can be reduced to reutilizable products onlywith difficulty, which has left the actual reutilization of carbondioxide in the realm of theory or in the academic field to date.

Natural carbon dioxide degradation proceeds, for example, viaphotosynthesis. This involves conversion of carbon dioxide tocarbohydrates in a process subdivided into many component steps overtime and, at the molecular level, in terms of space. As such, thisprocess cannot easily be adapted to the industrial scale. No copy of thenatural photosynthesis process with photocatalysis on the industrialscale to date has had adequate efficiency.

An alternative is the electrochemical reduction of carbon dioxide.Systematic studies of the electrochemical reduction of carbon dioxideare still a relatively new field of development. Only in the last fewyears have there been efforts to develop an electrochemical system thatcan reduce an acceptable amount of carbon dioxide. Research on thelaboratory scale has shown that electrolysis of carbon dioxide may beaccomplished using metals as catalysts. The publication “ElectrochemicalCO₂ reduction on metal electrodes” by Y. Hori, published in: C. Vayenas,et al. (eds.), Modern Aspects of Electrochemistry, Springer, New York,2008, p. 89-189, discloses Faraday efficiencies at different metalcathodes; see table 1. If carbon dioxide is reduced, for example, atsilver, gold or zinc cathodes, what is formed is almost exclusivelycarbon monoxide.

TABLE 1 Electrode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.3 25.55.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.00.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.50.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.188.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.00.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.00.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.00.0 0.0 0.0 0.0 99.7 99.7

The table gives Faraday efficiencies [o] of products that form in thecarbon dioxide reduction at various metal electrodes. The valuesreported apply to a 0.1 M potassium hydrogen-carbonate solution aselectrolyte and current densities below 10 mA/cm².

At a silver cathode, for example, predominantly carbon monoxide and onlya little hydrogen form. The reactions at anode and cathode can berepresented by the following reaction equations:

Cathode: 2 CO₂+4 e⁻+4 H⁺→2 CO+2 H₂O

Anode: 2 H₂O→O₂+4 H⁺+4 e⁻

As can also be inferred from table 1, at a copper cathode for instance,a multitude of hydrocarbons are formed as reaction products. One aspectof particular economic interest is, for example, the electrochemicalproduction of methane or ethylene, ethanol or monoethylene glycol. Theseare higher-energy products than carbon dioxide.

Ethylene: 2CO₂+12e⁻+8H₂O→C₂H₄+12OH⁻

Methane: CO₂+8e⁻+4H₂O→CH₄+4OH⁻

Ethanol: 2CO₂+12e⁻+9H₂O C₂H₅OH+12OH⁻

Monoethylene glycol: 2CO₂+10e⁻+8H₂O→HOC₂H₄OH+10OH⁻

With a chloride-containing electrolyte, the following reaction canproceed at the anode:

2 Cl⁻→Cl₂+2 e⁻

In the electrochemical conversion of matter of carbon dioxide to ahigher-energy product, there is an interest in increasing the economicviability, and in improvement with regard to the continuous operabilityof the electrolysis systems.

SUMMARY

Consequently, an improved solution for the electrochemical utilizationof carbon dioxide would avoid the disadvantages known from the priorart. More particularly, the solution may enable continuous carbondioxide conversion. The teachings of the present disclosure may providean improved reduction process and electrolysis system for carbon dioxideutilization.

For example, some embodiments may include electrolysis systems forcarbon dioxide utilization, comprising an electrolyzer (E1-E5) having ananode (A) in an anode space (AR) and a cathode (K) in a cathode space(KR). The cathode space (KR) has at least one entrance for carbondioxide (CO₂) and is configured to bring the carbon dioxide (CO₂) thathas entered into contact with the cathode (K). The cathode space (KR)comprises or can accommodate a catholyte which can enter the cathodespace (KR) through the same entrance or a separate entrance and whichincludes alkali metal cations. The anode space (AR) has at least oneentrance for an anolyte and comprises an anolyte or can accommodate itvia this entrance, wherein the anolyte includes chlorine anions.

In some embodiments, there is a deposition tank (AB), wherein thedeposition tank (AB) is configured for crystallization of an alkalimetal hydrogencarbonate and/or alkali metal carbonate out of thecatholyte and has a product outlet (PA3).

In some embodiments, the deposition tank (AB) has a cooling apparatus.

In some embodiments, at least one reservoir (PR) is configured andarranged with connection to the cathode space (KR) and/or the depositiontank (AB) such that it serves to buffer the catholyte.

In some embodiments, the catholyte comprises at least one solvent,especially water.

In some embodiments, the anolyte includes at least one water-solublealkali metal salt.

In some embodiments, the anode space (AR) is connected to a gasseparation unit for separation of chlorine gas from the anolyte.

In some embodiments, anode space (AR) and cathode space (KR) areseparated from one another by a cation-conducting membrane (M).

As another example, some embodiments may include a reduction process forcarbon dioxide utilization by means of an electrolysis system asdescribed above. In some embodiments, a catholyte and carbon dioxide(CO₂) are introduced into a cathode space (KR) and brought into contactwith a cathode (K). Carbon dioxide (CO₂) is reduced at the cathode (K).An anolyte including chloride anions (Cl⁻) is introduced into an anodespace (AR) and brought into contact with an anode (A). Chloride anions(Cl⁻) are oxidized at the anode (A) to chlorine (Cl₂) and the latter isseparated from the anolyte as chlorine gas by means of a gas separationunit. The anolyte includes alkali metal cations that migrate into thecatholyte. At least a portion of the catholyte volume is introduced intoa deposition tank, where an alkali metal hydrogencarbonate and/or alkalimetal carbonate crystallizes out.

In some embodiments, there is reduction at the cathode (K) of carbondioxide (CO₂) to carbon monoxide (CO), ethylene (C₂H₄), methane (CH₄),ethanol (C₂H₅OH) and/or monoethylene glycol (OHC₂H₄OH).

In some embodiments, the hydroxide ions (OH⁻) formed in the carbondioxide reduction are converted to hydrogencarbonate ions (HCO₃ ⁻) withcarbon dioxide (CO₂) present in excess.

In some embodiments, at least a portion of the catholyte volume isintroduced into a deposition tank, where it is cooled down by at least15 kelvin, preferably at least 20 kelvin.

In some embodiments, at least a portion of the catholyte volume isintroduced into a deposition tank, where the pH thereof is lowered fromabove 8 to a pH of 6 or less by blowing in carbon dioxide (CO₂).

In some embodiments, at least a portion of the catholyte volume isintroduced into a deposition tank, where an alkali metalhydrogencarbonate is crystallized and is subsequently converted to analkali metal carbonate by heating.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples and embodiments of teachings of the present disclosure aredescribed in an illustrative manner with reference to FIGS. 1 to 6 ofthe appended drawing:

FIG. 1 shows a schematic diagram of an electrolysis system with a carbondioxide reservoir and deposition tank, according to teachings of thepresent disclosure;

FIG. 2 shows a schematic diagram of an electrolysis system with a gasdiffusion electrode, according to teachings of the present disclosure;

FIG. 3 shows a schematic diagram of a PEM setup of an electrolysis cell,according to teachings of the present disclosure;

FIG. 4 shows a schematic diagram of a PEM half-cell coupled to a gasdiffusion electrode, according to teachings of the present disclosure;

FIG. 5 shows a schematic diagram of a PEM half-cell coupled to a cathodewith backflow, according to teachings of the present disclosure; and

FIG. 6 shows a Hagg diagram.

DETAILED DESCRIPTION

The electrolysis system of the present disclosure may allow improvedcarbon dioxide utilization. Some embodiments include at least oneelectrolyzer having an anode in an anode space and a cathode in acathode space. The cathode space has at least one entrance for carbondioxide and is configured to bring the carbon dioxide that has enteredinto contact with the cathode. In addition, the cathode space comprisesa catholyte or is configured to be able to accommodate a catholyte. Thecatholyte can access the cathode space through the same entrance as thecarbon dioxide or via a separate second entrance. At least the anodespace also includes alkali metal cations in the operation of the cellanode and cathode space.

Catholyte refers to an electrolyte which directly affects the cathode inthe electrolysis. Correspondingly, reference is also made hereinafter toanolyte when referring to an electrolyte directly affecting the anode inan electrolysis. Alkali metal cations refer to positively charged ionshaving at least one element of the first main group of the PeriodicTable.

The anode space of the electrolyzer has at least one entrance for ananolyte and comprises an anolyte or is at least configured toaccommodate an anolyte via this entrance, said anolyte includingchlorine anions.

In some embodiments, in the electrode system, the anode space and thecathode space are separated from one another by a membrane. The membranehere may include at least one mechanically separating layer, for examplea diaphragm, which separates the electrolysis products formed in theanode space and cathode space from one another. They could then also bereferred to as separator membrane or separation layer.

Since the electrolysis products are in many cases gaseous substances,the membrane may have a high bubble point of 10 mbar or higher. The“bubble point” is a defining parameter for the membrane used, whichdescribes the pressure difference ΔP between the two sides of themembrane from which gas flow through the membrane would set in. Themembrane may also be a proton- or cation-conducting or -permeablemembrane. While molecules, liquids or gases are being separated, protonor cation flow from the anode space to the cathode space is assured. Insome embodiments, the membrane comprises sulfonatedpolytetrafluoroethylene, e.g. Nafion.

In some embodiments, the electrolysis system further comprises at leastone deposition tank for crystallization of an alkali metalhydrogencarbonate and/or alkali metal carbonate out of the catholyte. Insome embodiments, this deposition tank has a product outlet. Accordingto the product, whether an alkali metal hydrogencarbonate and/or alkalimetal carbonate is to be taken from the catholyte, and according to thealkali metal, a second deposition tank may also be provided for acrystallization process. The latter is then typically arrangeddownstream of the first deposition tank in catholyte circulationdirection.

According to the cathode material used, the reduction of carbon dioxidegives rise to different products: for example, carbon monoxide,ethylene, methane, ethanol, or monoethylene may be formed. In all thesecases, hydroxide ions also form, which may be neutralized tohydrogencarbonate by excess carbon dioxide. The source of the alkalimetal cations is in the anode space. A cation stream through themembrane compensates for the electrical current resulting from thevoltage applied.

For example, the alkali metal cations and the chloride anions may bemetered into the anolyte in the form of a chloride salt. While thechloride anions are oxidized at the anode to chlorine and leave theanolyte circuit as chlorine gas, the alkali metal cations migratethrough the membrane into the catholyte circuit, where they react in thecathode space with the carbonate or hydrogencarbonate formed there togive an alkali metal carbonate or alkali metal hydrogencarbonate and mayleave the catholyte circuit via the separate product outlet of thedeposition tank.

The electrolysis systems of the present disclosure may have produce notonly chlorine but at least one alkali metal carbonate and/or alkalimetal hydrogencarbonate as a chemical material of value. Whether alkalimetal carbonate or alkali metal hydrogencarbonate is formed depends, forexample, on the alkali metal and the utilization method. In aqueoussolution, for instance, the solubility is crucial. The sparingly solublecarbonate or hydrogencarbonate crystallizes out. In the case of sodiumand potassium it is the hydrogencarbonate that is more sparingly solublethan the carbonate, and then has to be calcined in a subsequent step.The combustion of sodium in carbon dioxide is an example in which carbonmonoxide and, in a direct manner, sodium carbonate Na₂CO₃ are produced.

Furthermore, the electrolysis system may utilize carbon dioxide and itis thus also typically possible to provide at least one third materialof value, for example carbon monoxide, ethylene, methane, ethanol ormonoethylene glycol. The exploitation of the compensating current of thecations thus creates an electrolysis system which enables continuoushydrogencarbonate production.

As already described, the actual cathode reaction in which the carbondioxide is reduced is followed by a subsequent reaction, namely theneutralization of the hydroxide ions (OH—). These are especiallyneutralized by excess carbon dioxide to hydrogencarbonate (HCO₃ ⁻). Thisfirstly has the effect that the pH in the cathode space is thus bufferedwithin a pH range from 6 to 8. It also has the effect that theelectrolyte concentration rises considerably. But if the catholyte isconducted into a catholyte circuit, i.e. pumped into the cathode spaceand led out of it again, the hydrogencarbonate formed in the cathodespace can be taken from the catholyte. For this purpose, moreparticularly, at least one pump in each case may be arranged in thecatholyte circuit, or else, for example, in the anolyte circuit, andthis ensures electrolyte circulation.

Subsequent to the neutralization reaction of the hydroxide ions (OH⁻)with excess carbon dioxide to give hydrogencarbonate ions (HCO₃ ⁻),these may react further with alkali metal cations to give alkali metalhydrogencarbonates. The alkali metal cations present in the cathodespace come from the anode space, into which they were initiallyintroduced especially in the form of alkali metal chloride as oxidationreactant or in the form of another alkali metal salt, for increasing theconductivity for example. The alkali metal cations in the anode spacemay be replenished in the form of alkali metal chloride. The membranebetween the anode space and cathode space may be chosen such that thecation flow from the anode space toward the cathode in the electricalfield of the electrolyzer is assured. The effect of the temperature andalso pH dependence of the solubility of alkali metal hydrogencarbonatesis then that different processes for crystallization or for withdrawalfrom the catholyte are undertaken:

Firstly, it is possible to utilize the temperature dependence of thesolubility of the alkali metal hydrogencarbonates desired aselectrolysis product. For this purpose, the deposition tank may comprisea cooling apparatus, by means of which the catholyte is cooled down byseveral degrees Kelvin compared to the temperature range that prevailsin the electrolyzer. In some embodiments, the temperature difference setfrom the deposition tank to the electrolyzer is at least 15 K,especially at least 20 K. According to the electrolyte concentration inthe catholyte and according to the alkali metal cations with which thehydrogencarbonate is formed, a temperature difference between 30 K and50 K may also be particularly suitable. The temperature differencebetween the electrolyzer and deposition tank may be within a temperaturerange between 5 K and 70 K.

The lowering of the temperature in the deposition tank may providecooling in the catholyte circuit precedes the recycling of the catholyteinto the cathode space. Thus, excessively high systemic evolution ofheat, specifically in the electrolyzer, is avoided. But it is alsopossible to dispense with any cooling unit provided specially for thispurpose.

In the case of the deposition method of crystallization of the alkalimetal hydrogencarbonate by means of cooling of the catholyte too,preference is given to using pH buffers provided, for example, in abuffer reservoir to the deposition tank and/or to the catholyte circuitand/or to the cathode space, to correspondingly buffer the catholytevolume.

The pH of the catholyte can also be employed as such for the control ofthe operation of deposition of the alkali metal hydrogencarbonate out ofthe electrolyte. For this purpose, more particularly, the pH in thecathode space is at first kept at a higher value, for example 8 orhigher. This can shift the equilibrium in favor of the alkali metalcarbonate and away from the alkali metal hydrogencarbonate. Forcrystallization in the deposition tank, the pH is then lowered, e.g., toa value of 6 or less, which leads to formation and crystallization ofthe alkali metal hydrogencarbonate. The lowering of the pH is typicallyaccomplished by blowing carbon dioxide into the deposition tank.

According to the alkali metal cation with which the hydrogencarbonatereacts, and depending on the pH in the cathode space, it is at firstpossible to form an alkali metal hydrogencarbonate or an alkali metalcarbonate. More particularly, the two procedures described forwithdrawal of the desired product from the catholyte can also becombined. In some cases, for example in the case of formation of sodiumhydrogencarbonate NaHCO₃, it is also possible, for example, to obtainthe sodium carbonate Na₂CO₃ subsequently from the sodiumhydrogencarbonate NaHCO₃ that has crystallized out by heating. In thatcase, hydrogencarbonate may be first produced and deposited, andsubsequently the desired proportion thereof is processed further to givecarbonate.

The pH dependence of the hydrogencarbonate or carbonate ions is shown,for example, in FIG. 6 in a Hagg diagram for a sodium carbonatesolution.

In the electrolysis system, a buffer reservoir may be provided in theanolyte circuit, which can especially also serve for introduction orreplenishment of alkali metal chloride into the electrolyte, to maintainthe salt content in the anolyte.

In some embodiments, the catholyte includes at least one solvent,especially water. Typically, aqueous electrolytes and correspondinglywater-soluble conductive salts may be employed. The conductive saltcontent can be increased by the addition of further carbonates,hydrogencarbonates, but also sulfates or other conductive salts, toincrease the conductivity of the electrolyte in the catholyte circuitand also in the anolyte circuit, which leads to an increase in theconversion of matter in the overall system. According to which and whatamounts of additional conductive salts are present in the catholytecircuit, the crystallization process is adjusted correspondingly toextract the desired product with maximum purity. Conductive salts usedmay be chosen such that the solubility thereof differs significantlyfrom that of the alkali metal hydrogencarbonate or the alkali metalcarbonate.

Typically, the electrolysis system has a gas separation unit on theanolyte side, which is configured to undertake the separation ofchlorine gas from the anolyte. In the catholyte circuit too, a gasseparation unit may be provided, for example when it is directed tocarbon monoxide gas production via use of a silver-containing cathode.In the anolyte circuit and in the catholyte circuit, additional unitsfor inlets or outlets from the system or additional buffer reservoirsmay be provided.

The nature and quality of the membrane used in the electrolyzerultimately makes a significant contribution to how pure the crystallizedproduct is. If the membrane used is merely a separator, it is alsopossible, for example, for chloride anions to diffuse into the cathodespace, even counter to the electrical field in the electrolyzer, suchthat not only hydrogencarbonate but possibly also chlorides are formed.Therefore, in some embodiments, there is a cation-conducting membranethrough which virtually exclusively cations can pass. A purelyanion-conducting membrane may be less useful.

In some embodiments, the reduction process described for carbon dioxideutilization by means of an electrolysis system as described abovecomprises the following steps: a catholyte and carbon dioxide areintroduced into a cathode space, where they are contacted with acathode. Within the cathode space, this catholyte includes alkali metalcations which migrate through the membrane that separates anode spaceand cathode space. At least a portion of the catholyte volume may beintroduced into a deposition tank, where an alkali metalhydrogencarbonate and/or an alkali metal carbonate crystallizes out.

In some embodiments, an anolyte including chloride anions, is broughtinto contact with an anode. The chloride anions are oxidized at theanode to chlorine and the latter is separated from the anolyte aschlorine gas by means of a gas separation unit. Typically, thisreduction process is effected such that anolyte and catholyte are eachconducted into a separate circuit, meaning that two pumps are providedin the electrolysis system, which bring about transport of the catholytethrough the cathode space and transport of the anolyte through the anodespace at least at one point in the circuit.

The circuits are separated from one another by the membrane in theelectrolyzer, which may permit exclusively transport of cations from theanode space into the cathode space. More particularly, the alkali metalcations required in the cathode space may be obtained from the anodespace. For this purpose, the anolyte may include an alkali metalchloride; the latter may be used as conductive salt, or else likewise aselectrolysis reactant. In some embodiments, the alkali metal chloride inthe anolyte can be used as electrolysis reactant, and an additionalconductive salt, for example a sulfate, a phosphate et cetera, e.g, analkali metal sulfate, can be used. In some embodiments, it is alsopossible to use ammonium salts or homologs thereof. Imidazolium salts orother ionic liquids can have a positive effect on the selectivity of theelectrode, particularly the cathode.

In some embodiments, in the reduction process, the reduction of thecarbon dioxide at the cathode produces carbon monoxide, ethylene,methane, ethanol and/or monoethylene glycol. For this purpose, anappropriate cathode may be used as catalyst for these reactions. Forthis purpose, the cathode may include copper. In some embodiments, thisreduction process produces, in addition to carbon dioxide utilization,chemical substances of value.

In some embodiments, the hydroxide ions formed in the carbon dioxidereduction can be converted to hydrogencarbonate ions with carbon dioxidepresent in excess. Hydrogencarbonate production directly in the cathodespace allows these to react further directly with alkali metal cationspresent in the cathode space to give a further material of value whichis of interest, which would otherwise have to be produced in separateproduction processes. In some embodiments, to withdraw this material ofvalue from the system, at least a portion of the catholyte volume isintroduced into a deposition tank, where it is cooled down by at least15 K, and/or by at least 20 K. Here, the temperature dependence of thecarbonate solubility is thus exploited to withdraw the material of valuefrom the catholyte circuit. The temperature differential from depositiontank to electrolyzer may also be more than 30 K, especially also morethan 50 K, according to the present alkali metal hydrogencarbonate to beextracted and also depending on which further salts are present in thecircuit. The temperature differential between electrolyzer anddeposition unit may be between 5 K and 70 K. In some embodiments, forextraction of the hydrogencarbonate product from the catholyte volume,the dependence of the solubility on the pH is exploited. This processcan be combined with the temperature-dependent process.

In some embodiments, for this purpose, at least a portion of thecatholyte volume is introduced into a deposition tank, where the pHthereof is lowered, especially by means of blowing in carbon dioxide,from above 8 to a pH of 6 or less. Specifically the buffering of the pHto a value of more than 8 in the cathode space prevents theprecipitation of the alkali metal hydrogencarbonate in the cathode spaceitself.

In some embodiments, the reduction process can be undertaken such thatthe precipitated alkali metal hydrogencarbonate is converted to alkalimetal carbonate by heating. This can be effected directly after thecrystallization of the hydrogencarbonate in the deposition tank orseparately from the electrolysis system described.

In some embodiments, as an alternative to the temperature method ofcrystallization or to the temperature-assisted crystallization, or elsein combination therewith, the process can also be run such that the pHin the cathode space is kept at the upper limit of the reaction ofaround 8 or higher, such that the equilibrium is at first shifted infavor of sodium carbonate:

2 NaHCO₃→Na₂CO₃+H₂O+CO₂.

For this purpose, the carbon dioxide supply to the system must be verywell controlled, to arrive at and maintain this basic regime. In thedeposition tank, the pH would then be lowered for optimal deposition ofthe sodium hydrogencarbonate by blowing in carbon dioxide, and hence theequilibrium reaction would again be shifted in favor of sodiumhydrogencarbonate.

However, the process is not restricted to sodium hydrogencarbonate. Forexample, it is also possible to prepare potassium hydrogencarbonate inthis process. Analogously to the deposition process described for sodiumhydrogencarbonate, it is also possible to crystallize the potassiumhydrogencarbonate out of a pure potassium hydrogencarbonate electrolyteby lowering the temperature in the deposition tank. At 20° C. thesolubility of potassium hydrogencarbonate is 337 g/l, and at 60° C. itis 600 g/l.

A somewhat different procedure is necessary if an additional conductivesalt, for example potassium sulfate (K₂50₄), is to be used. This has alower solubility of 111.1 g/l at 20° C. and 250 g/l at 100° C., whichmeans that the potassium sulfate would always precipitate out first inthe mixed electrolyte. In order to obtain the potassiumhydrogencarbonate (KHCO₃) from an electrolyte containing both potassiumsulfate and potassium hydrogencarbonate, it is necessary to proceed asfollows: in the deposition tank AB, potassium sulfate K₂50₄preferentially crystallizes out and can be fed back to the electrolytesubsequently, i.e. downstream of the deposition tank AB in circulationdirection. The electrolyte volume from which the potassium sulfate K₂SO₄has already been removed is then concentrated, preferably in a furtherdeposition tank, meaning that the water is removed from the potassiumhydrogencarbonate solution, for example by cooling, to obtain thecrystalline material.

In principle, this process is also applicable to other cations ormixtures of cations. The migration of the cations results inconcentration of the catholyte to such an extent that the most sparinglysoluble salt or double salt separates out. It is important here that theprocess of concentration and deposition does not proceed in the cathodespace, i.e. not in the electrolysis cell itself, but that the catholyteis transported for the purpose into a deposition tank integrated withinthe electrolysis system. By means of a further additional physical orchemical difference between the electrolysis cell and deposition tank,i.e., for example, by means of a temperature, pH or pressure gradient,the deposition in the deposition tank is achieved or promoted. Asuitable pressure differential between electrolysis cell and depositiontank may be up to 100 bar. A pressure differential between 2 bar and 20bar would preferably be chosen. An elevated pressure in the depositiontank would promote hydrogencarbonate formation.

On the anode side, in principle, alternative anode reactions are alsoconceivable, but coupling to chlorine production is the mosteconomically viable, since the chlorine market is about 75 millionmetric tons per year. Current production of sodium hydrogencarbonate(NaHCO₃) is about 50 million metric tons per year, which have to datebeen produced via the energetically unfavorable Solvay process.

With the electrolysis system and reduction process described, it ispossible to electrochemically, continuously and simultaneously producethree materials of value: at the cathode, a material of value such ascarbon monoxide, ethylene, methane, ethanol or monoethylene glycol isobtained from the carbon dioxide reduction, sodium hydrogencarbonateand/or sodium carbonate is co-produced as a conversion product of thisreduction reaction formed in the cathode space, and on the anode sidechlorine is produced.

FIGS. 1 and 2 show, in a schematic representation, examples ofelectrolysis systems for carbon dioxide reduction, which can equally beread as flow diagrams for the reduction process described. Shown on theleft-hand side in each case is the anolyte circuit AK, and on theright-hand side the catholyte circuit KK. These two circuits AK, KK areconnected via the electrolyzer E1, E2, the anode space AR and cathodespace KR of which are connected to one another and separated from oneanother by means of a membrane M. The membrane M used may be acation-conducting membrane M. In the anode space AR is disposed an anodeA, and in the cathode space KR a cathode K, which are electricallyconnected by a voltage source U.

Each of the circuits AK, KK may include a pump P1, P2, which pump theelectrolytes through the electrolyzer. In addition, units N1, N2, N3 inthe two circuits AK, KK may be present at different points in the flowdirection, which may be additional inlets or outlets or in the form ofbuffer reservoirs. In the anolyte circuit AK, at least one gasseparation unit G2 with a product outlet PA2 is provided, by means ofwhich the chlorine gas product Cl₂ can be withdrawn. Likewise providedin the catholyte circuit KK is at least one gas separation unit G1 witha product outlet PA1, by means of which, for example, the carbonmonoxide electrolysis product CO, and, for example, hydrogen H₂ as wellcan be withdrawn. But it is also possible for further electrolysisproducts, such as ethylene, methane, ethanol, monoethylene glycol, to bewithdrawn from the system via this or, for example, via a furtherproduct outlet. The electrolyzer E1, E2 has, for example, a gasdiffusion electrode GDE for the carbon dioxide inlet.

In the case of the electrolyzer E1 shown in FIG. 1, a two-chamber setupis chosen and the carbon dioxide CO₂ is introduced into the electrolytevia a reservoir CO₂—R and upstream of the cathode space KR incirculation direction. The catholyte circuit KK, in both cases shown,has a deposition tank AB which may be incorporated directly into thecircuit or through which just a portion of the catholyte volume isconducted. For this purpose, as shown in FIGS. 1 and 2, a branch in thecircuit KK may be provided. The deposition tank AB or a plurality ofseries-connected deposition tanks may be connected, for example, to acooling unit or to a buffer reservoir PR, such that the crystallizationof the hydrogencarbonate is promoted by establishing a temperaturedifferential, pressure differential or pH differential with respect tothe electrolyzer E1, E2. In addition, the deposition tank AB may includea product outlet PA3. Multiple series-connected deposition tanks wouldeach have a product outlet.

FIGS. 1 and 2 thus show electrolysis systems usable for the methodsdescribed herein. In this setup, it is ensured that there are separateanolyte circuits AK and catholyte circuits KK. The electrolytes used arethen pumped continuously through the electrolysis cell E1, E2, i.e.through the anode space AR and through the cathode space KR. For thispurpose, in the setup, one pump P1, P2 is provided in each of the twocircuits AK, KK. The setup may include materials made of plastic,plastic-coated metal or glass. Reservoir vessels used may be glassflasks; the cell itself is made, for example, of PTFE, and the hoses ofneoprene.

The electrolyzer E1, E2, as constructed in the electrolysis systemsshown, may also have a different setup as shown, for example, in FIGS. 3to 5. An alternative electrolysis cell is that according to the polymerelectrolyte membrane setup (PEM setup). In this case, at least oneelectrode directly adjoins the polymer electrolyte membrane PEM.Correspondingly, the electrolysis cell can be configured as a PEMhalf-cell, as shown in FIGS. 4 and 5, in which the anode side isconfigured as a PEM half-cell, i.e. the anode A is arranged in directcontact with the membrane PEM and the anode space AR is arranged on theside of the anode A facing away from the membrane.

In the cases as shown in FIGS. 4 and 5, the cathode K is porous and atleast partly gas-permeable and/or electrolyte-permeable. In FIG. 4, theanode PEM half-cell is combined with a gas diffusion electrode GDE forintroducing the carbon dioxide CO₂ into the cathode space KR. Also shownin FIG. 5 is a cathode K with backflow, the cathode space KR of which isconnected to a gas reservoir via the cathode K. The gas reservoir here,for its part, has at least one gas inlet GE and optionally a gas outletGA. Such an embodiment has been used to date, for example, as anoxygen-depolarized electrode, for example in the production of sodiumhydroxide solution. In that case, there would be oxygen backflow throughthe cathode K. The oxygen-depolarized cathode can be used, for example,to avoid hydrogen formation H₂ in the cathode space KR in favor of areaction to give water H₂O. The energy of water formation here lowersthe necessary system voltage U and thus brings about lower energyconsumption of the electrolysis system. Since the cathode K of anoxygen-depolarized electrode consists primarily of silver, it can alsocatalyze carbon dioxide reduction. If no oxygen is provided, theoxygen-consuming reaction cannot proceed. Instead, carbon dioxidereduction to carbon monoxide CO takes place with a certain degree ofhydrogen formation.

If, for example, sodium is chosen as alkali metal, in the case of use ofa copper-containing cathode K, the following reactions proceed in thecathode space KR:

Ethylene: 12 NaCl+14 CO₂+8 H₂O→C₂H₄+12 NaHCO₃+6 Cl₂

Methane: 8 NaCl+9 CO₂+4 H₂O→CH₄+8 NaHCO₃+4 Cl₂

Ethanol: 12 NaCl+14 CO₂+9 H₂O→C₂H₅OH+12 NaHCO₃+6 Cl₂

Monoethylene Glycol:

10 NaCl+12 CO₂+8 H₂O→HOC₂H₄OH+10 NaHCO₃+5 Cl₂.

In the case of a silver-containing cathode K, the following reactionswould proceed at the cathode:

Carbon Monoxide:

2 NaCl+3 CO₂+H₂O→CO+2 NaHCO₃+Cl₂.

These equations describe the cumulative process in the electrolysiscell. The chlorine gas Cl₂ forms, as described, through oxidation of thechloride anions Cl⁻ at the anode A; the other electrolysis products format the cathode K or through conversion reactions in the cathode spaceKR.

The example of sodium may be suitable since sodium hydrogencarbonate canbe deposited very efficiently from the electrolyte. Moreover, sodiumhydrogencarbonate and sodium carbonate are important chemical materialsof value that are frequently required. Global annual sodium carbonateproduction is about 50 000 000 metric tons, as can be inferred forexample from the Roskill market report “Soda Ash: Market Outlook to2018”, available from Roskill Information Services Ltd, E-Mail:info@roskill.co.uk, www.roskill.co.uk/soda-ash.

The solubility of sodium hydrogencarbonate NaHCO₃ in water H₂O iscomparatively low and also shows strong temperature dependence; seetable 2.

TABLE 2 Molecular formula KHCO₃ K₂SO₄ K₃PO₄ KI KBr KCl NaHCO₃ Na₂SO₄Molar mass 100.1 174.3 212.3 166.0 119.0 74.6 84.01 142.04 in g/molSolubility in H₂O at 20° C.: In g/l 337 111 900 1400 678 344 96 170 Inmol/l 3.37 0.64 4.24 8.43 5.70 4.61 1.19 1.14 Conductivities σ in mS/cm:At 0.05M 4.8 9.9 17.3 7.2 7.7 7.4 5.8 14.8 At 0.1M 9.1 19.2 30.1 14.014.3 13.8 28.1 51.6 At 0.5M 38.9 (69.9) 108 65.2 67.5 62.8

Table 2 lists further salts, potassium hydrogencarbonate KHCO₃,potassium sulfate K₂SO₄, potassium phosphate K₃PO₄, potassium iodide KI,potassium bromide KBr, potassium chloride KCl, sodium hydrogencarbonateNaHCO₃, sodium sulfate Na₂SO₄, which can be used with preference. Butother sulfates, phosphates, iodides or bromides can also be used toincrease the conductivity in the electrolyte. By constantly supplyingthe carbon dioxide, it is not necessary to supply carbonates orhydrogencarbonates; instead, they are formed in operation in the cathodespace KR.

The solubility of sodium hydrogencarbonate NaHCO₃ in water is 69 g/l at0° C., 96 g/l at 20° C., 165 g/l at 60° C. and 236 g/l at 100° C. Sodiumcarbonate NaCO₃, by contrast, has comparatively good solubility; thesolubility thereof is 217 g/l at 20° C. With continuing electrolysis,the sodium hydrogencarbonate NaHCO₃ thus has a tendency to crystallizeout in the electrolysis cell E1, E2. This can be counteracted via anelevated temperature as arises as a result of the operation of thesystem, and also via corresponding buffering of the pH.

The sodium hydrogencarbonate NaHCO₃ is not supposed to crystallize outof the electrolyte until within the deposition tank AB. As a result ofthe pumped circulation of the electrolyte in a circuit KK, the sodiumhydrogencarbonate NaHCO₃ formed in the cathode space KR is conducted outof it and the catholyte circuit KK can run through a deposition tank AB,or a part-volume of the catholyte is branched into a deposition tank ABin which, for example, the sodium hydrogencarbonate NaHCO₃ crystallizesout as a result of the cooling of the electrolyte and can thus berecovered. Since the electrolysis cells E1, E2 are in any case heatedsignificantly in operation as a result of process losses, there can beeffective crystallization at temperature differentials of up to 70 Kbetween cathode space KR and deposition tank AB. Preference is given toworking within a range between temperature differential 30 K and 50 K.Especially with a temperature differential of at least 15 K or even atleast 20 K.

If the catholyte also contains further additions for enhancingconductivity and hence increasing the energy efficiency, and anadditional conductive salt thus minimizes the ohmic losses in theelectrolyte, this has to be taken into account in the crystallization ofthe sodium hydrogencarbonate NaHCO₃, in order to obtain a product ofmaximum purity. In some embodiments, a hydrogensulfate HSO₄ ⁻ or sulfateSO₄ ²⁻ is included as a conductive additive. This may, for example, besodium sulfate Na₂SO₄ or sodium hydrogensulfate NaHSO₄. The solubilityof sodium hydrogensulfate NaHSO₄ is 1080 g/l at 20° C. and that ofsodium sulfate Na₂SO₄ is 170 g/l at 20° C.; see table 2. Given thisgreat difference in solubility from sodium hydrogencarbonate NaHCO₃, itis assured that sodium hydrogencarbonate NaHCO₃ will crystallize outpreferentially in the deposition tank.

This variant of the reduction process may basically replace the Solvayprocess that has been used as standard to date for sodiumhydrogencarbonate production. This is because the Solvay process forsodium hydrogencarbonate production has a great disadvantage, namelythat it consumes very large amounts of water. Moreover, for everykilogram of soda, i.e. sodium carbonate Na₂CO₃, about one kilogram ofunusable calcium chloride CaCl₂ is also produced, which is usuallyreleased into the wastewater and hence into rivers and seas. Given anannual production of 50 million metric tons of sodium carbonate Na₂CO₃,this is thus about 50 million metric tons of calcium chloride CaCl₂.

The natural sources for soda Na₂CO₃ that are available aside from theSolvay process are by no means sufficient. Sodium hydrogencarbonateNaHCO₃ occurs as the natural mineral nahcolite in the United States ofAmerica. It usually occurs in fine distribution in oil shale and canthen be produced as a by-product of oil production. Particularly richnahcolite horizons are being mined in the state of Colorado. However,annual production in 2007 was only 93 440 metric tons. It also occurs,for example, in soda lakes in Egypt, in Turkey in Lake Van, in EastAfrica, for example in Lake Natron and other lakes in the East Africanrift, in Mexico, in California (USA), and as trona(Na(HCO₃).Na₂CO₃.2H₂O) in Wyoming (USA), Mexico, East Africa and in thesouthern Sahara.

FIG. 6 shows, for illustration of the dependence on the concentrationand pH parameters, an example of a Hagg diagram of a 0.05 molar solutionof carbon dioxide CO₂. Within a moderate pH range, carbon dioxide CO₂and salts thereof are present alongside one another. While carbondioxide CO₂ under strongly basic conditions preferentially takes theform of carbonate CO₃ ²⁻ and preferentially takes the form ofhydrogencarbonate HCO₃ ⁻ in the moderate pH region, thehydrogencarbonate ions are driven out of the solution in the form ofcarbon dioxide CO₂ at low pH values in an acidic medium.

What is claimed is:
 1. An electrolysis system for carbon dioxideutilization, the system comprising: an electrolyzer including an anodein an anode space and a cathode in a cathode space; the cathode spacehas an entrance for carbon dioxide; the cathode space comprises acatholyte including alkali metal cations; the anode space has anentrance for an anolyte; the anode space comprises an anolyte comprisingchlorine anions.
 2. The electrolysis system as claimed in claim 1,further comprising a deposition tank configured for crystallization ofan alkali metal hydrogencarbonate and/or alkali metal carbonate out ofthe catholyte; wherein the deposition tank includes a product outlet. 3.The electrolysis system as claimed in claim 2, further comprising acooling apparatus for the deposition tank.
 4. The electrolysis system asclaimed in claim 2, further comprising a reservoir connected to thecathode space or the deposition tank to buffer the catholyte.
 5. Theelectrolysis system as claimed in claim 1, wherein the catholytecomprises a solvent.
 6. The electrolysis system as claimed in claim 1,wherein the anolyte includes at least one water-soluble alkali metalsalt.
 7. The electrolysis system as claimed in claim 1, furthercomprising the anode space connected to a gas separation unit forseparation of chlorine gas from the anolyte.
 8. The electrolysis systemas claimed in claim 1, further comprising a cation-conducting membraneseparating the anode space and cathode space from one another.
 9. Areduction process for carbon dioxide utilization, the processcomprising: introducing a catholyte and carbon dioxide into a cathodespace with a cathode; reducing carbon dioxide at the cathode;introducing an anolyte including chloride anions into an anode space andbrought into contact with an anode; wherein the anolyte includes alkalimetal cations that migrate into the catholyte; oxidizing chloride anionsat the anode to chlorine; separating chlorine from the anolyte aschlorine gas using a gas separation unit; and introducing at least aportion of the catholyte volume into a deposition tank, where an alkalimetal hydrogencarbonate and/or alkali metal carbonate crystallizes out.10. The reduction process as claimed in claim 9, further includingreducing, at the cathode, carbon dioxide (CO₂) to carbon monoxide (CO),ethylene (C₂H₄), methane (CH₄), ethanol (C₂H₅OH), and/or monoethyleneglycol (OHC₂H₄OH).
 11. The reduction process as claimed in claim 10,further comprising converting the hydroxide ions (OH⁻) formed in thecarbon dioxide reduction to hydrogencarbonate ions (HCO₃ ⁻) with carbondioxide (CO₂) present in excess.
 12. The reduction process as claimed inclaim 9, further including introducing at least a portion of thecatholyte into a deposition tank, where it is cooled down by at least 15kelvin.
 13. The reduction process as claimed in claim 9, furthercomprising introducing at least a portion of the catholyte volume into adeposition tank, where the pH thereof is lowered from above 8 to a pH of6 or less by blowing in carbon dioxide (CO₂).
 14. The reduction processas claimed in claim 9, further comprising introducing at least a portionof the catholyte volume into a deposition tank, where an alkali metalhydrogencarbonate is crystallized and is subsequently converted to analkali metal carbonate by heating.